1. Field of the Invention
The present invention relates generally to the field of scroll compressors, and more particularly relates to a shunt pulsation trap for reducing gas pulsations and induced vibration, noise and harshness (NVH), and improving compressor off-design efficiency.
2. Description of the Prior Art
A scroll compressor (also called scroll pump and scroll vacuum pump) is a device for compressing air, gas or refrigerant. It is used in air conditioning and refrigeration, as an automobile supercharger and as a vacuum pump. A scroll compressor operating in reverse is known as a scroll expander, and can be used to generate mechanical work from the expansion of a fluid, compressed air or gas. Many residential central heat pump and air conditioning systems and a few automotive air conditioning systems employ a scroll compressor instead of the more traditional rotary, reciprocating, and wobble-plate compressors.
A scroll compressor consists of a stationary scroll, which has a discharge port at the center, and an orbiting scroll that revolves around the stationary scroll without rotating around its own axis. The gas is first sucked into the compression pockets from the peripheral side of the scroll. Then the gas is compressed as the volume of the trapped pockets becomes decreased, and is released near the center of the scrolls to a discharge port to finish the cycle. It is essentially a positive displacement mechanism but using an orbiting scroll instead of a reciprocating piston so that displacement motion can be much faster without experiencing any shaking forces. The result is a more continuous and smoother stream of flow with a more compact size and replacing the traditional reciprocating or rolling piston types.
It has been well known that scroll compressors generate gas pulsations at discharge due to inherently possessing a fixed-compression ratio. The pulsation amplitudes are especially significant under high pressure conditions as in air conditioning and refrigeration or for operating under either an under-compression or an over-compression when pressure at the discharge port is either greater or less than the pressure of the compressed gas pocket just before the opening. According to the conventional theory, an under-compression produces a rapid backflow of the gas into the pocket while an over-compression causes a rapid forward flow of the gas from the pocket. These flow pulsations are periodic in nature and very harmful if left undampened, such as inducing noises and exciting structural and system vibrations.
To lessen the problem, a pulsation dampener typically in the form of a large volume chamber, is required at the discharge side of a scroll compressor to dampen the gas borne pulsations. But its effectiveness is limited for gas pulsation control and produces other problems like inducing structural vibrations and exciting noises of other frequencies. At the same time, a more effective pulsation dampening as used today often creates more pressure losses that reduce compressor overall efficiency that suffers already at off-design conditions like an under-compression or an over-compression. So with the ever demanding energy conservation and stringent NVH regulations from the government plus growing public awareness of the comfort level in residential and office applications, there is more and more an urgent need for quieter and more efficient scroll compressors.
In addition to the commonly used serial discharge dampener, a skewed porting method using a flow equalizing strategy is disclosed in U.S. Pat. No. 5,370,512 to Fujitani et al. The idea, say for under-compression as an example, is to feed back a portion of the outlet gas through an enlarged leakage slot to the compression chamber prior to discharging to the outlet, thereby gradually increasing the gas pressure inside the gas pocket, hence reducing discharge gas pressure spikes when compared with a sudden opening at discharge. However, its effectiveness for gas pulsation attenuation is limited in practice to only 5-10 dB reduction, not enough for today's demands from both the market and the general public. Moreover, compressor efficiency suffers due to enlarged leakage area from skewed porting as reported.
It is against this background that prompts a new gas pulsation theory by the present inventor postulating that a composition of large amplitude waves and induced fluid flow under the off-design conditions (an under-compression or an over-compression) are the primary causes of high gas-borne pulsations and low efficiency. The new gas pulsation theory is based on a well studied physical phenomenon as occurs in a classical shock tube (invented in 1899 by French scientist Pierre Vieille) where a diaphragm separating a region of high-pressure gas p4 from a region of low-pressure gas p1 inside a closed tube. As shown in FIGS. 1A-1B, when the diaphragm is suddenly broken, a series of expansion waves is generated propagating to the high-pressure p4 region at the speed of sound, and simultaneously a series of pressure waves which quickly coalesces into a shockwave is propagating to the low-pressure p1 region at a speed faster than the speed of sound. Between the oppositely travelled shock wave and expansion waves, a unidirectional flow is induced in the same direction as the shockwave but travels at a slower velocity ΔU. The interface separating low and high pressure gases, referred to as the contact surface, travels at the same velocity ΔU as the induced flow.
By analogy, the sudden opening of the diaphragm separating the high and low pressure gases in a shock tube is just like the sudden opening of the compressed gas pocket to discharge port under off-design conditions, because both are transient in nature and driven by the same forces from a suddenly exposed pressure difference. In this way, the well established results of the Shock Tube theory accumulated over the past 100 years can be readily applied to examine hence reveal the gas pulsation mechanism of a scroll type compressor or expander.
To understand the gas pulsation generation mechanism, a cycle of a classical scroll compressor as illustrated in FIG. 3A is examined by following one flow pocket marked dark in the illustration (in reality, two pockets are formed symmetrically as two scrolls are engaged with each other). In suction phase in FIG. 3A, low pressure gas first enters circumferentially the spaces between spirals of a pair of orbiting and stationary scrolls from the peripheral side of the scroll. Then gas becomes trapped in a crescent-shaped pocket as it is moved to the center and simultaneously being compressed as the trapped volume between the spirals decreases as shown in trapping and compression phases from FIG. 3A. The discharging phase shows the moment that the compressed gas is suddenly opened to the discharge port. A serial dampener, typically a large volume chamber located right after the discharge port, is commonly employed to attenuate pulsations generated in the gas stream as shown in the dampening phase before flows out to a downstream pipe.
According to the conventional theory when the pocket is opened to the discharge port in case of an under-compression, a backflow would rush into the pocket compressing the gas and equalizing the pressure inside the pocket with the discharge pressure. Since it is almost instantaneous and there is no volume change taking place inside the pocket, the compression is regarded as a constant volume process, or an iso-choric process that inherently consumes more work compared with an internal adiabatic compression (as indicated on P-V diagram by the additional “horn” area).
However, in light of the shock tube theory, the discharging phase as shown in FIG. 3A resembling the diaphragm bursting of a shock tube as shown in FIG. 1B would generate a composition of pressure waves (due to 3D effects and limited pocket size inside scroll compressor, these pressure waves may not be able to coalesce into a real shockwave as taking place in an one-dimensional long shock tube), expansion waves and induced flow. The pressure wave front sweeps through the low pressure gas inside the pocket and compresses it at the same time at the speed of sound as in case of the under-compression. While for the over-compression, a fan of expansion waves would sweep through the high pressure gas inside the pocket and expand it at the same time at the local speed of sound. This results in an almost instantaneous adiabatic wave compression or expansion well before the induced flow interface (backflow as in conventional theory) could arrive because wave travels much faster than the fluid, as illustrated by the wave propagation pattern in FIGS. 3C-3D. In this view, the pressure waves are the primary driver for the compression as in case of the under-compression while the backflow is simply an induced flow behind the pressure waves after compression takes place. Moreover, as the pressure waves travel to low pressure pocket as shown in FIG. 3C, a simultaneously generated expansion wave front travels in the opposite direction causing rapid pressure reduction and inducing a rapid backflow down-stream. It is believed, by the new theory, that this expansion wave front and the accompanying induced back flow are the main sources of gas pulsations experienced at discharge port for a scroll compressor during an under-compression. While for the over-compression, the gas pulsations at discharge are a composition of the pressure wave front and induced forward flow into the pipe downstream, as the simultaneously generated expansion waves travel into the high pressure pocket as shown in FIG. 3D. Any effective pulsation control should address all of these bi-directional waves and induced unidirectional flow at the same time while minimizing potential flow losses in the process.
Based on this new insight, the pre-opening to discharge as disclosed by Fujitani et al is predicted to be able to reduce gas pulsations, to a degree, by feeding back part of the gas fluid to elongate the discharging time. However, it failed to recognize hence attenuate the simultaneously generated expansion or pressure waves at the pre-opening that eventually would travel down-stream unblocked, causing high gas pulsations. Moreover, the prior art failed to address the high flow losses associated with the high induced fluid velocity through the serial dampener and discharging process, resulting in low compressor off-design efficiency.
The theory underlining the present invention can be summarized into the following Pulsation Rules for industrial applications because the large amplitude of most of the industrial gas pulsations that far exceed the upper limit of 140 dB of the classical Acoustics would invalidate the small disturbance assumption and the use of linearized wave equation. The Pulsation Rules are intended as a simplified way to answer some fundamental questions of gas pulsations such as: What is the physical nature of gas pulsations? What exactly triggers them to happen? Where and when are they generated and how to predict quantitatively their behaviors at source such as amplitude, travelling direction and speed? In principle, these rules are applicable to different gases and for gas pulsations generated by any industrial PD type gas machinery or devices such as engines, expanders, or pressure compressors, vacuum pumps, or even for pulsations generated by valves say in a pipe line.                1. Rule I: For any two divided compartments, either moving or stationery, with different gas pressures p1 and p4, there will be no or little gas pulsations generated if the two compartments stay divided (isolated).        2. Rule II: If, at an instant, the divider between the high pressure gas p4 and the low pressure gas p1 is suddenly removed in the direction of divider surface, gas pulsations are instantaneously generated at the location of the divider and at the instant of the removal a composition of a fan of Compression Waves (CW) or a quasi-shockwave, a fan of Expansion Waves (EW) and an Induced Fluid Flow (IFF) with magnitudes as follows:CW=p2−p1=p1[(p4/p1)1/2−1]=(p4×p1)1/2−p1   (1)EW=p4−p2=CW*(p4/p1)1/2=p4−(p4×p1)1/2   (2)ΔU=(p2−p1)/(ρ1×W)=CW/(ρ1×W)   (3)Where ρ1 is the gas density at low pressure region, W is the speed of the lead compression wave, ΔU is the velocity of Induced Fluid Flow (IFF);        3. Rule III: Pulsation component CW is the action by the high pressure (p4) gas to the low pressure (p1) gas while pulsation component EW is the reaction by low pressure (p1) gas to high pressure (p4) gas in the opposite direction, and their magnitudes are such that they approximately divide the pre-trigger pressure ratio p4/p1, that is, p2/p1=p4/p2=(p4/p1)1/2. At the same time, CW and EW pair together to induce the third pulsation component, a unidirectional fluid flow IFF in a fixed relationship of CW-IFF-EW.        
Rule I implies that there would be no or little pulsations during the suction, transfer and compression (expansion) phases of a scroll cycle because of the absence of either a pressure difference or a sudden opening. The focus instead should be placed upon the discharge phase, especially at the moment when the discharge port is suddenly opened and during off-design conditions like either an under-compression, UC (over-expansion, OE) or over-compression, OC (under-expansion, UE).
Rule II indicates specifically the location and the moment of pulsation generation are at the discharge and at the instant the discharge port suddenly opens. Moreover, it defines two sufficient conditions for gas pulsation generation:                a) The existence of a pressure difference p4−p1;        b) The sudden opening of the divider separating the pressure difference p4−p1.        
Because a scroll compressor or an expander converts energy between shaft and fluid by dividing incoming continuous fluid stream into parcels of pocket size and then discharges each pocket separately at the end of each cycle, there always exists a “sudden” opening at discharge phase to return the discrete fluid parcels back to a continuous fluid stream again. So both sufficient conditions are satisfied at the moment of the discharge opening if scroll compressors and expanders operate at the off-design points such as UC (OE) or OC (UE).
Rule II also reveals the composition and magnitudes of gas pulsations as a combination of large amplitude Compression Waves (CW) or a quasi-shockwave, a fan of Expansion Waves (EW) and an Induced Fluid Flow (ΔU). These waves are non-linear waves with ever changing wave form during propagation. This is in direct contrast to the acoustic waves that are linear in nature and wave fronts stay the same and do not induce a mean through flow. It is also noted that the three different pulsations (CW, EW and IFF) are generated as a whole simultaneously and one cannot be produced without the others. This makes gas pulsations very difficult to control because it's not one but all three effects have to be dealt with.
Rule III shows further that the interactions between two gases of different pressures are mutual so that for every CW pulsation, there is always an equal but opposite EW pulsation in terms of pressure ratio (p2/p1=p4/p2). Together, they induce a unidirectional fluid flow pulsation (IFF) in the same direction as the compression waves (CW).
Accordingly, it is always desirable to provide a new design and construction of a scroll compressor that is capable of achieving significant gas pulsation and NVH reduction at source and improving compressor off-design efficiency while being kept compact in size and suitable for quiet, efficient and variable pressure ratio applications at the same time.